1. Field of the Invention
The present invention relates to a semiconductor memory device, a method of manufacturing the same, and a method of screening the same, and more particularly to a nonvolatile semiconductor memory device comprising a variable resistance element as a memory element, for example.
2. Description of the Related Art
In order to improve memory device densities with the same design rule (F), there is a scheme of decreasing the area necessary to form one bit. Similarly, by forming the same memory configuration in a vertically stacked configuration, the memory device densities can also be improved. A cross-point-type memory cell is an example of a way to achieve them. According to this configuration, since a storage region is formed in the area where the upper interconnect and the lower interconnect cross each other, a cell area per one bit can be decreased. Further, its simple configuration is suitable for vertically stacking layers.
A cross-point-type memory cell can be memory-operated by being combined with a variable resistance element such as a phase-change random access memory (PCRAM) and a resistive random access memory (ReRAM). By rewriting/determining the resistance value of the memory element interposed between the upper interconnect and the lower interconnect, binary 0 and 1 can be stored/read. In this case, in order to prevent an error when reading memory data, a diode with a rectifying function, as well as a memory element, is usually added to the memory element.
A PIN diode is a representative diode used as a memory cell of a cross-point-type memory cell. The PIN diode is formed using silicon. By forming an intrinsic semiconductor layer, into which impurities are not introduced, between an N-type semiconductor layer and a P-type semiconductor layer, a reverse leakage current is suppressed, and a withstand voltage in a reverse direction is improved. When the thickness of the intrinsic semiconductor layer increases, the characteristics in the reverse direction are improved and the forward current decreases. Further, when the impurity concentration of the N-type semiconductor layer and the P-type semiconductor layer increases, the forward current increases because of decrease in resistance, but the heat during the process causes impurities to diffuse into the intrinsic semiconductor layer and the characteristics in the reverse direction degrade. Thus, the characteristics in the forward and reverse directions are in trade-off relationship with regard to changes in various configurations or processes.
A document (Jpn. Pat. Appln. KOKAI Publication No. 2008-78663) discloses a structure of a phase change memory device that does not degrade in data retention properties even at high temperature.